Piezoelectric ceramics are commonly being used as sensors, actuators and transducers because of their strong electromechanical coupling effect.
A detection/test system, which combines such sensors, actuators, transducers with feedback or feed-forward control circuitry, is an important technology for many industry and military applications. One particular application is the active control of vibrations. For example, active control of the vibration inside the body of an airplane can greatly reduce the noise in the passenger cabin. Active control of the vibration of the wings can greatly reduce the damping by airflow and thus increase the efficiency of the airplane. Relatedly, active control of the vibration of a submarine can greatly reduce the acoustic noise it generates and thus greatly reduce its chance of being detected. Another application of detection/test systems is real-time structural health monitoring. For example, embedded sensors and transducers in a structure can produce in-site detection of cracks in the structures and thus predict and assist in avoiding critical failure of the structure.
A significant drawback of piezoelectric ceramics is that it is difficult to make a thin, large sheet (at many inches to several feet scale), due to the brittle nature of the material. Due to this limitation, it cannot be mounted to a curved surface or embedded in a structure which needs to be flexible. Unfortunately, many real world applications require detecting and testing of curved surfaces and/or flexible structure, thus the mentioned brittleness greatly limits the applications of piezoelectric ceramic materials in detection/test systems.
An alternative is to use piezoelectric polymers which are flexible and can be manufactured in large scale. Unfortunately, the piezoelectric effect of piezoelectric polymers is weak—about one-tenth of piezoelectric ceramics—and the materials are very soft.
One path taken to develop a detector/test system is represented by research at Stanford University and which is coined as the Stanford Multi-Actuator-Receiver Transduction Layer (SMART layer). Particularly, a manufacturing method has been proposed for integrating a network of distributed piezoceramic actuators/sensors onto laminated carbon/epoxy composite structures. The network of built-in actuators/sensors is used to monitor the health of the host composite structure by acquiring information about the condition of the structure throughout its life. The manufacturing method applies a printed circuit board technique to fabricate a thin flexible layer with a network of piezoceramics. It is used as an extra ply that is either inserted into or bonded onto the surface of a composite laminate to give it actuating and sensing capabilities. More particularly, the system implements the use of a flexible printed circuit, commonly referred to as “Flex.” The proposed concept used the Flex technique to make a large, thin flexible layer that contains a network of distributed piezoceramics connected by printed circuits.
However, the fabrication techniques for the SMART layer are labor intensive and restrictive in design choices. Particularly, the disclosed fabrication process for the SMART layer do not lend itself to obtaining of a flexible tape with high density elements and a variety of geometric shapes for those elements, which in turn permits more versatile functional capabilities. It also does not consider use of elements within a thickness range of about 10 μm, or greater, formed by a direct marking technology.